1. Field of the Invention
This invention relates generally to variable gain circuits and, more particularly, is directed to a variable gain circuit of the type having a gain which is varied exponentially with respect to a control voltage supplied thereto.
2. Description of the Prior Art
When using variable gain circuits in, for example, audio noise reduction circuits, it is generally required that high performance variable gain circuits be used. Accordingly, variable gain circuits of the type using bipolar junction transistors are frequently employed in such noise reduction circuits. With such arrangement, the exponential voltage-current characteristic of the bipolar junction transistors provides that the gain of the variable gain circuit is varied exponentially with respect to a control voltage supplied thereto.
Such variable gain circuits of the exponential type are particularly advantageous for use with noise reduction circuits when constructed as monolithic bipolar integrated circuits. In particular, such monolithic bipolar integrated circuits provide substantially perfect thermal coupling and compatibility of the characteristics of the components thereof. However, it becomes extremely difficult, if not impossible, to provide conventional transistors having conductivities which are in substantially perfect complement to each other with such monolithic integrated circuits. As a result, lateral transistors are employed as PNP transistors in the monolithic integrated circuit, but such lateral PNP transistors are markedly inferior to conventional PNP transistors as to the current amplification factor and cut-off frequency.
In order to overcome such drawbacks, a voltage controlled variable gain circuit has been devised in which a differential amplifier comprised of bipolar junction transistors having the emitters thereof commonly supplied with a constant current from a constant current source, is driven by the output of an operational amplifier to which an input signal is supplied. First and second current divider circuits are provided, each comprised of two bipolar junction transistors having the emitters thereof commonly connected to the collector of a respective transistor of the differential amplifier and with the output signal from one transistor of each current divider circuit being fed back to the input of the operational amplifier. The output signal of the variable gain circuit is produced in response to the output signals from the other transistors of the first and second current divider circuits. With this arrangement, the current gain of the circuit is represented as an exponential function of the control voltage supplied to the first and second current divider circuits, and the overall characteristics of the circuit are governed essentially by the properties of the transistors used in the first and second current divider circuits. Since NPN transistors with good operating characteristics are used for such transistors, optimum characteristics of the circuit are produced, and the circuit becomes particularly suitable for production as a monolithic integrated circuit.
However, with the latter circuit, the constant current supplied to the emitters of the transistors used in the differential amplifier must be set to a value larger than the maximum possible value of the sum of the input and output currents to and from the circuit. On the other hand, it is to be appreciated that the steady state currents of the current divider circuits which are supplied by the transistors of the differential amplifier are always equal to one-half the value of the constant current. As a result of such large steady state currents, shot noise in the current divider circuits is undesirably increased. Further, the current dividing ratios of the current divider circuits fluctuate in response to heat noise caused by the ohmic resistances of the transistors used in the current divider circuits, the degree of such fluctuations depending on the magnitude of the steady state currents, in the same manner as the shot noise.
In addition, in the case where an offset voltage exists in the current divider circuits, or where a gain deviation exists in current inverter circuits at the outputs of the current divider circuits, an offset component is generated in the output current of the variable gain circuit, even when the input current supplied thereto is equal to zero, the magnitude of the offset component being dependent on the control signal supplied to the variable gain circuit. As a result, a feedthrough component representative of variations in the control signal will appear in the output signal, with the magnitude of the feedthrough component also being dependent on the steady state currents through the current divider circuits.